Asahi/Pentax 55mm f1.8 "Zebra" Lens.
I was given this lens sometime back by a friend of mine who got it from his dad. It's an M42 mount from the mid 60s. I didn't have anything to mount it to, so it sat on the shelf for a while. I looked at it last week and noticed it had an adapter on it to mount to K-Mount. I just happen to have gotten a K-Mount to E-Mount adapter a couple days earlier for my SMC 50mm 1.7 lens, so i threw it on! Here's a few pics:
This version of the lens is apparently radioactive!